Klaus has been waiting to hear Maud's voice for hours now.
When he made Maud his Death Death, Klaus knew exactly what he was doing...
As the heir of death, Klaus has the means to select his Twelve Agents of Death. These 12 people will carry an aspect of death.
Maud is Death's heart.
She possesses the Emotion of Death.
Basically, she grows stronger by feeding on the dying emotions of all beings.
From the start of the battle, Klaus told her to gather the souls of the dead; however, it was merely a lie meant to trick the heavens.
What Maud had been doing all this time was gathering the resentment of all the dead warriors on both her side and the enemy side, waiting for when she could form the first Heart of Death.
Just now, when Klaus knew he was about to have a very nasty welcome now that the main powerhouse in the battle had revealed himself, Maud also informed him she was done with the mission he had given her.
'Ah, I like it when plans come together in the end.'
